If we're using a homebrewed gcc-4.2, xcrun may fail to find it (or,
worse, find superenv's shim instead). Explicitly add it to the PATH and
search all path elements for the requested tool.

Also make sure to specify 'gcc-4.2' as the compiler name, not plain
'gcc'. That can resolve to llvm-gcc and to gcc-4.0 on various Xcodes.

Closes Homebrew/homebrew#17935.

This fixes superenv on Xcode 3.2.6, and also fixes C++ software with
gcc-4.2 on all Xcodes.

Fixes Homebrew/homebrew#20080.

Due to the precedence of "and" relative to "||", this was not working as
intended; but because #version influences #prefix, the outcome was still
correct. So we can simplify this method quite a bit, and take the
opportunity to share code with #prefix.

Fixes Homebrew/homebrew#20084.

Closes Homebrew/homebrew#20081.

This matches the eql? definition for requirements.

Fixes Homebrew/homebrew#20045.